Analyzing the function of song and its evolution as a communication signal provides an essential backdrop for understanding the physiological and neural mechanisms responsible for song learning, perception, and production. The reverse also is true-understanding the mechanisms underlying song learning provides insight into how song has evolved as a communication signal. Song has two primary functions: to repel other males from a defended space and to attract females and stimulate their courtship. The developmental stress hypothesis we present here builds on studies of the development of the song system to suggest how learned features of song, including complexity and local dialect structure, can serve as indicators of male quality useful to females in mate choice. The link between song and male quality depends on the fact that brain structures underlying song learning largely develop during the first few months post-hatching and that during this same period, songbirds are likely to be subject to nutritional and other developmental stresses. Individuals faring well in the face of stress are able to invest more resources to brain development and are expected to be correspondingly better at song learning. Learned features of song thus become reliable indicators of male quality, with reliability maintained by the developmental costs of song. Data from both field and laboratory studies are now beginning to provide broad support for the developmental stress hypothesis, illustrating the utility of connecting mechanistic and evolutionary analyses of song learning.